Several publications are referenced in this application by numerals in parenthesis in order to more fully describe the state of the art to which this invention pertains. Full citations for these references are found at the end of the specification. The disclosure of each of these publications is incorporated by reference herein.
Tobacco cultivars, such as Nicotiana tabacum Xanthi nc, which carry a dominant disease resistance gene (e.g., N gene) have been propagated. Infection of these plants with tobacco mosaic virus (TMV) results in a hypersensitive response. The hypersensitive response is characterized by host cell death and necrosis at the site of infection and restriction of pathogen growth and movement. Following induction of the hypersensitive response, systemic acquired resistance develops throughout the plant. Establishment of systemic acquired disease resistance results in enhanced resistance to a secondary challenge by the same and some unrelated pathogens.
An increasing body of evidence suggests that salicylic acid (SA) is an important component of the signal transduction pathway(s) leading to local and systemic plant defense responses, including the activation of pathogenesis-related (PR) proteins (1-5). Earlier studies suggest that one mechanism of SA action is to inhibit catalase and ascorbate peroxidase, thereby elevating endogenous H.sub.2 O.sub.2 levels and generating salicylate free radicals (6-8; Chen and Klessig, unpublished data). The increased levels of H.sub.2 O.sub.2 and/or products of reactions induced by salicylate free radicals such as lipid peroxide may act as signals for the activation of plant defense genes such as the PR-1 gene. However, little is known about the molecular mechanisms beyond these signals. Therefore, to facilitate the production of transgenic plants with enhanced resistance to certain pathogens, it is essential to identify other important genetic signaling components. The present invention relates to the identification and characterization of such components and provides methods for the generation of transgenic plants expressing these components. The transgenic plants so generated exhibit enhanced disease resistance to TMV and other plant pathogens.